The invention relates to semipermeable membranes based on organically modified silicic-acid polycondensates, to a process for preparing them and to their use in gas exchange and in separation techniques, especially in gas separation, dialysis, pervaporation, and micro-, ultra- and hyperfiltration. The membranes of the invention are flat or tubular membranes.
For the separation of mixtures of substances a very wide variety of membrane materials are known, all of them being capable of improvement in terms of their serviceableness and their economics. For instance, known membrane materials, such as cellulose acetate, have poor temperature and pressure stability and swell severely in organic solvents. A consequence of the poor temperature stability, pressure stability and solvent resistance is that pore size changes continually under service conditions and consequently may result in nonreproducible results and in short membrane service lives.
Ultrafiltration, for example, is carried out predominantly in an aqueous system, so that the requirements of the membranes used for the ultrafiltrations, in terms of mechanical and thermal stability (sterilizability to 140xc2x0 C.), resistance to acids and alkali""s, and customizable hydrophilic/hydrophobic properties are particularly stringent. The polymers used to date to produce membranes are unable to meet these requirements at the same time and, for example, their relatively good thermal stability up to about 140xc2x0 C. is accompanied by a lack of sufficient mechanical stability.
The surface modifications often necessary for various applications (adjustment of porosity, adsorption behavior, etc.) necessitate further, subsequent processes. Furthermore, materials modified in this way have the disadvantage of having only a modified monolayer at the surface, so making them extremely sensitive to mechanical and chemical exposure.
Commercially customary polymers such as polyethylenes, polypropylenes, polysulfones, polyimides, polymethacrylates, etc. have poor gas permeability (e.g., to O2, CO2, etc.). An increase in permeation on the basis of these polymer types is possible only by incorporating pores. For example, hollow polymer fibers provided with a defined, interconnecting porosity are obtainable directly only by highly complex spinning processes or by subsequent and thus additional process steps. Where polymers modified in this way are used, for example, for gas exchange in fluid systems, there exists the risk of passage of the fluid phase. For example, in the case of O2/CO2 exchange in the blood in oxygenators during operations on the open thorax, the pores harbor a considerable hazard potential. In the case of relatively long operations in particular, the passage of blood through the pores is observed fairly frequently.
Very high gas permeation values without porosity are realizable only with highly specific polymers (silicones, substituted polysilyipropynes, etc.). The high gas permeability is achieved, however, only at the expense of extreme reductions in the mechanical properties. As permeability increases there is a reduction in strength and modulus of elasticity, i.e., the material becomes increasingly softer. Self-supporting thin films and stable hollow fibers of low wall thickness are therefore not possible. Films and hollow fibers having a degree of permeability which can be established over wide ranges are possible only on the basis of very different types of polymer in conjunction with different production techniques.
Membranes based on silicic-acid heteropolycondensates exhibit excellent resistance to acids and organic solvents and are also highly stable in the pH range up to about 10.
From DE 27 58 415 C2 it is known to process silicic-acid heteropolycondensates to porous membranes by mechanically cutting the polycondensates, which are obtained in compact blocks, into very thin slices which are then usedxe2x80x94directly or after being ground beforehandxe2x80x94as membranes. Since, however, the silicic-acid heteropolycondensates are generally not sufficiently elastic, the membrane slices break on cutting, and also the necessary membrane surface area is usually not achieved by this method.
DE 29 25 969 C2 describes another process for producing porous membranes on the basis of silicic-acid heteropolycondensates at the interface between an organic and an aqueous phase. Since, however, the resulting membranes have a high water content owing to the contact with an aqueous phase, the risk exists of excessive shrinkage on drying, and of associated cracking. The hydrolytic polycondensation of the starting components to silicic-acid heteropolycondensates proceeds with substance egress, so that shrinkage of the polycondensates takes place unavoidably.
With the membranes described in DE 27 58 415 C2 and DE 29 25 969 C2, the shaping operation to form the membrane, and its curing, take place by means of an inorganic condensation, i.e., by the construction of an Sixe2x80x94Oxe2x80x94Si network. These membranes have very poor mechanical properties; the mechanical stabilities only rarely satisfy the requirements made of them. Furthermore, these membranes are brittle and inflexible.
EP 0094060 B1 discloses a further process for producing membranes on the basis of silicic-acid heteropolycondensates. In that process polycondensation is carried out on the surface of a support which supports the membrane. Here again, the operation of shaping to form the membrane, and its curing, take place by inorganic condensation, i.e., by the construction of an inorganic network. The resultant membranes are supported and not self-supporting.
It is an object of the present invention to provide semipermeable membranes for gas exchange and for separations, the exchange capacity of said membranes being variable over wide ranges and adapted to the requirements of the particular application. It is another object to provide membranes that may be supported but also that may be self-supporting and in a tubular or flat form. It is yet another object to provide membranes having the permeability and flexibility that can be varied over wide ranges and adapted to the requirements of a particular application. Yet another object is to provide membranes that combine high mechanical stability with high permeability, particularly with respect to gases, inter alia, to allow use for gas exchange and for separation without risking penetration of the fluid phase. Still further, even with high permeation values, such membranes may remain self-supporting. Another object is to provide membranes that are toxicologically acceptable and thus suitable for use in the medical sector.
It is a further object of the present invention to provide a process to manufacture semipermeable membranes having properties that can vary over wide ranges. In embodiments of the process, variation of the process steps allow control of the chemical and physical properties of the membrane, within wide ranges, to the requirements of particular applications. Moreover, embodiments of the process are simple, rapid and inexpensive to carry out. By means of the process it should be possible to manufacture membranes which meet the above-mentioned requirements. Furthermore, the process should also be suitable for the continuous production of hollow fibers and flat membranes. In addition, the surface modifications which are often necessary for various applications, for example, in order to avoid blood coagulation, in order to adjust the polarity, adsorption characteristics, etc., should be realizable both during the synthesis of the material, i.e., in situ, and also subsequently.
One embodiment of the invention is a process for producing a semipermeable membrane, comprising forming a semipermeable membrane from a low-viscosity to resinous liquid produced by hydrolytic polycondensation of a material comprising at least one compound selected from the group consisting of:
a compound of formula I 
wherein
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R1=alkylene, arylene, arylenealkylene or alkylenearylene comprising between 0 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R2=alkylene, arylene, arylenealkylene or alkylenearylene comprising between 0 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R3=hydrogen, R2xe2x80x94R1xe2x80x94R4xe2x80x94SiXxR3xe2x88x92x, carboxyl, alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R4=xe2x80x94(CHR6xe2x80x94CHR6)nxe2x80x94, where n=0 or 1, xe2x80x94CHR6xe2x80x94CHR6xe2x80x94Sxe2x80x94R5xe2x80x94, xe2x80x94COxe2x80x94Sxe2x80x94R5xe2x80x94, xe2x80x94CHR6xe2x80x94CHR6xe2x80x94NR6xe2x80x94R5xe2x80x94, xe2x80x94Yxe2x80x94CSxe2x80x94NHxe2x80x94R5, xe2x80x94Sxe2x80x94R5, xe2x80x94Yxe2x80x94COxe2x80x94NHxe2x80x94R5xe2x80x94, xe2x80x94COxe2x80x94Oxe2x80x94R5xe2x80x94, xe2x80x94Yxe2x80x94COxe2x80x94C2H3(COOH)xe2x80x94R5xe2x80x94, xe2x80x94Yxe2x80x94COxe2x80x94C2H3(OH)xe2x80x94R5xe2x80x94 or xe2x80x94COxe2x80x94NH6xe2x80x94R5xe2x80x94,
R5=alkylene, arylene, arylenealkylene or alkylenearylene comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R6=hydrogen, alkyl or aryl having 1 to 10 carbon atoms,
R7=hydrogen, alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
X=hydrogen, halogen, hydroxyl, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl or NRxe2x80x32, where Rxe2x80x3=hydrogen, alkyl or aryl,
Y=xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NR6xe2x80x94,
Z=xe2x80x94Oxe2x80x94 or xe2x80x94(CHR6)mxe2x80x94, where m=1 or 2,
a=1, 2 or 3, where b=1 if a=2 or 3,
b=1,2 or 3, where a=1 if b=2 or 3,
c=1 to 6,
x=1, 2 or 3, where a+x=2, 3 or 4;
(ii) a compound of formula II 
wherein
B=a straight-chain or branched organic radical having at least one Cxe2x95x90C double bond and 4 to 50 carbon atoms,
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
R3=alkylene, arylene, arylenealkylene or alkylenearylene comprising between 0 to 10 carbon atoms, wherein any of these radicals optionally is interrupted by an atom or group selected from the group consisting of oxygen atom, sulfur atom, and amino group,
X=hydrogen, halogen, hydroxyl, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl or NRxe2x80x32, where Rxe2x80x3=hydrogen, alkyl aryl or alkylaryl,
A=O, S or NH if d=1 and Z=CO and
R1=alkylene, arylene or alkylenearylene comprising between 1 to 10 carbon atoms, wherein any of these radicals optionally is interrupted by an atom or group selected from the group consisting of oxygen atom, sulfur atom, and amino group, and
R2=COOH or H,
or
A=O, S, NH or COO if d=1 and Z=CHRxe2x80x2, where
Rxe2x80x2=H, alkyl, aryl or alkylaryl, and
R1=alkylene, arylene or alkylenearylene comprising between 1 to 10 carbon atoms, wherein any of these radicals optionally is interrupted by an atom or group selected from the group consisting of oxygen atom, sulfur atom, and amino group, and
R2=OH,
or
A=O, S, NH or COO if d=0 and
R1=alkylene, arylene or alkylenearylene comprising between 1 to 10 carbon atoms, wherein any of these radicals optionally is interrupted by an atom or group selected from the group consisting of oxygen atom, sulfur atom, and amino group, and
R2=OH,
or
A=S if d=1 and Z=CO and
R1=N and
R2=H,
a=1, 2 or 3,
b=0, 1 or 2, where a+b=3,
c=1,2,3 or 4;
(iii) a compound of formula III
{XaRbSi[(Rxe2x80x2A)c](4xe2x88x92axe2x88x92b)}xBxe2x80x83xe2x80x83(III)
wherein
A=O, S, PRxe2x80x3, PORxe2x80x3, NHC(O)O or NHC(O)NRxe2x80x3,
B=a straight-chain or branched organic radical derived from a compound Bxe2x80x2 having at least one (if c=1 and A=NHC(O)O or NHC(O)NRxe2x80x3) or at least two Cxe2x95x90C double bond(s) and 5 to 30 carbon atoms,
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
Rxe2x80x2=alkylene, arylene or alkylenearylene,
Rxe2x80x3=hydrogen, alkyl, aryl or alkylaryl,
X=hydrogen, halogen, hydroxyl, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl or NRxe2x80x32,
a=1,2 or 3,
b=0, 1 or 2,
c=0 or 1,
x=an integer whose maximum value corresponds to the number of double bonds in the compound Bxe2x80x2 minus 1, or is equal to the number of double bonds in the compound Bxe2x80x2 if c=1 and A is NHC(O)O or NHC(O)NRxe2x80x3, wherein said alkyl and alkenyl radicals optionally are substituted straight-chain, branched or cyclic and comprise 1 to 20 carbon atoms, the aryl optionally is a substituted phenyl, naphthyl or biphenylyl, the alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl, alkylaryl, arylalkyl, arylene, alkylene and alkylenearyl radical is a derivative of said alkyl or aryl radical;
(iv) a compound of formula IV
YaSiXxR4xe2x88x92axe2x88x92xxe2x80x83xe2x80x83(IV)
wherein
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
X=hydrogen, halogen, hydroxyl, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl or NRxe2x80x32, where Rxe2x80x3=hydrogen, alkyl, aryl or alkylaryl,
Y=an organic radical having 1 to 30, preferably 1 to 20 carbon atoms and 1 to 5, preferably 1 to 4 mercapto groups,
a=1, 2 or 3,
x=1, 2 or 3, where a+x=2, 3 or 4;
and
(v) a precondensate derived from a compound shown represented in any of formulae I to IV
and wherein said hydrolytic polycondensation material further optionally comprises at least one compound selected from the group consisting of:
(i) a compound of formula V
XaSiR4xe2x88x92axe2x80x83xe2x80x83(V)
wherein
R=alkyl, alkenyl, aryl, alkylaryl or arylalkyl comprising between 1 to 15 carbon atoms, further optionally comprising an atom or group selected from the group consisting of oxygen atom, sulfur atom, ester, carbonyl, carboxyl, amido, and amino,
X=hydrogen, halogen, hydroxyl, alkoxy, acyloxy, alkylcarbonyl, alkoxycarbonyl or NRxe2x80x32, where Rxe2x80x3=hydrogen, alkyl, aryl or alkylaryl,
a=1, 2 or 3; and
(ii) a precondensate derived from a compound of formula V
wherein said hydrolytic polycondensation is conducted by adding a substance selected from the group consisting of water, a solvent, and a condensation catalyst, and wherein said molar ratio of the sum of the compound(s) of formulaes I, II, III and IV to the sum of compound(s) of formula V is between 1:0 and 1:20.
Another embodiment of the invention is a semipermeable membrane produced by the above process. Yet another embodiment is a process selected from the group consisting of gas separation, reverse osmosis, electrodialysis, dialysis, pervaporation, microfiltration, ultrafiltration and hyperfiltration, wherein said process comprises effecting a separation using the semipermeable membrane described herein. Yet additional embodiments readily will be understood from the disclosure.